1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting seismic data.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill or not (to avoid a dry well).
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 drags plural detectors 12. The plural detectors 12 are disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to by those skilled in the art as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are distributed along a slanted straight line 14 that makes a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 also drags a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector). The reflected acoustic wave 22b propagates upwardly and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 22c is reflected back towards the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution are known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by the detectors.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, the ghosts disturb the accuracy of the final image of the subsurface and for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis. Further, the actual measurements need to be processed for obtaining the correct position of the various parts (reflectors) of the subsurface. Such a processing method is the migration.
U.S. Pat. Nos. 4,353,121 and 4,992,992, the entire content of which is incorporated herein by reference, describe processing procedures that allow ghosts to be removed from recorded seismic data by using an acquisition device that includes a seismic streamer slanted at an angle (on the order of 2 degrees) to the surface of the water (slanted streamer).
Using slanted streamers, it is possible to achieve ghost suppression during data summation operation (during pre-stack operations). In fact, the acquired data are redundant, and the processing procedure includes a summation step or “stacking” for obtaining the final image of the subsurface structure from the redundant data. The ghost suppression is performed in the art during the stacking step because the recordings that contribute to the stack, having been recorded by different receivers, have notches at different frequencies, such that the information that is missing due to the presence of a notch on one seismic receiver is obtained from another receiver.
Further, U.S. Pat. No. 4,353,121 describes a seismic data processing procedure based on the following known steps: (1) common depth point collection, (2) one-dimensional (1D) extrapolation onto a horizontal surface, or “datuming”, (3) Normal MoveOut (NMO) correction, and (4) summation or stack.
Datuming is a processing procedure in which data from N seismic detectors Dn (with positions (xn, zn), where n=1, . . . N and N is a natural number, xi=xj but zi different from zj with i and j taking values between 1 and N), is used to synthesize data corresponding to seismic detectors that have the same horizontal positions xn and a same constant reference depth z0 for all the seismic detectors.
Datuming is called 1D if it is assumed that the seismic waves propagate vertically. In that case, the procedure includes applying to each time-domain recording acquired by a given seismic detector a delay or a static shift corresponding to the vertical propagation time between the true depth zn of a detector Dn and the reference depth z0.
Furthermore, U.S. Pat. No. 4,353,121 describes a summation of the primary (primary stack) by using the NMO correction that aligns the primaries, then a summation of the ghosts (ghost stack) by aligning the ghost reflections, and then combining the results of these two steps to obtain a post-stack image with a boosted signal-to-noise ratio.
Similar to U.S. Pat. No. 4,353,121, U.S. Pat. No. 4,992,992 proposes to reconstitute from seismic data recorded with a slanted cable seismic data as would have been recorded by a horizontal cable. However, U.S. Pat. No. 4,992,992, takes into account the non-vertical propagation of the seismic waves by replacing the 1D datuming step of U.S. Pat. No. 4,353,121 with a 2D datuming step. The 2D datuming step takes into account the fact that the propagation of the waves is not necessarily vertical, unlike what is assumed to be the case in the 1D datuming step proposed by U.S. Pat. No. 4,353,121.
More specifically, U.S. Pat. No. 4,992,992 reconstructs two sets of seismic data as if they had been recorded by a horizontal streamer and then sums the two sets after multiplication by a factor. The first set of data is synthesized by assuming that the seismic waves are propagating upward like the primary waves, and the second set is synthesized by assuming that the seismic waves are propagating downward like the ghosts. Upward propagation (rising wave) is defined by angles of propagation with respect to the horizontal between 0° and 180°, and downward propagation (descending wave) is defined by angles of propagation between 180° to 360° with the horizontal.
The methods described in U.S. Pat. Nos. 4,353,121 and 4,992,992 are seismic processing procedures in one dimension and in two dimensions. Such procedures, however, cannot be generalized to three dimensions. This is so because a sampling interval of the sensors in the third dimension is given by the separation between the streamers, on the order of 150 m, which is much larger than the sampling interval of the sensors along the streamers which is on the order of 12.5 m. Also, the existing procedures may apply a deghosting step at the beginning of the processing, which is not always very efficient.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, e.g., provide a 3D seismic processing procedure which allows imaging of the subsurface geology based on marine seismic data recorded at different water depths.